Recent trends in the design of passenger car engines make the use of steel camshafts advisable in engines equipped with roller type followers or valve lifters. The roller followers impose substantially higher compressive loads on the camshaft lobes. Accordingly, the uniformity of hardening of the cam lobes around their full peripheral extent is of utmost importance in order to resist lobe deformation and wear.
Heretofore, the manufacture of steel camshafts has generally entailed the formation of a forged steel blank which, after machining into general camshaft form, is hardened and then the cam lobes finished ground to precise surface profile. Such a steel camshaft manufacturing procedure, however, never attained any great degree of acceptance in the industry due to high manufacturing costs, low productivity levels, and the burning, cracking, and induced stresses caused by grinding the cam lobes to final shape after they have been hardened.
To overcome these problems, it has been recently proposed to produce steel camshafts by a so-called post grind hardening process wherein the cam lobes of the camshaft are first finished ground to their final surface profile, while the lobes are still in the soft or unhardened state, and the finished ground cam lobes then hardened to the requisite hardened depth. See SAE Technical Paper Series No. 860231 entitled "Post Grind Hardening, an Alternative Method of Manufacturing a Steel Roller Camshaft". Such a post grind hardening process, however, requires that the depth of hardening be kept to a minimum in order to ensure minimum heat distortion and maintain the accuracy of the finished ground cam lobes during the subsequent heating preparatory to hardening. For such reason, low heat input, short heat time, hardening methods for the cam lobes of the steel camshafts have been considered necessary by the industry.
Because of its relative simplicity as compared to other low heat input hardening methods such as electron beam and laser beam hardening, induction heating is generally preferred for the hardening of the cam lobes on steel camshafts. For this purpose, many in the industry have employed the prior induction heating devices which have been commonly used for hardening the cam lobes and also the fuel pump gear on cast iron type camshafts. Generally, the bearing surfaces of the conventional camshaft bearings have been left in an unhardened condition since they normally present a substantial bearing area for support by axially spaced bearing blocks.
The prior known induction heating and hardening devices conventionally employed for cast iron type camshafts have generally comprised an inductor in the form of a circular shaped heating coil within which the eccentrically contoured cam lobes of the camshaft are individually located, with the circular inductor encircling the cam lobe in closely spaced relation to form an inductive coupling gap therebetween which, because of the eccentric shape of the cam lobe, is of varying gap distance around the surface profile of the cam lobe. Energization of the inductor coil by a high frequency power supply of around 10 KHz and a relatively low power density at the cam lobe surface profile of, for instance, around 10 KW/inch.sup.2 for a period of a few seconds then effects inductive heating of the cam lobe. Although ordinarily unnecessary for most purposes, the camshafts may be rotated if desired during the inductive heating cycle to assure a uniform inductive heating of and thus a uniform hardening of the cam lobe around its entire surface profile on subsequent quenching of the heated cam lobe by a liquid quench medium directed thereagainst.
The need for uniformity and depth of hardening of the cam lobes of roller follower actuating steel camshafts around their full peripheral extent and minimal lobe deformation and wear, together with the desirability of using low heat input, short heat time, hardening methods for the finish ground cam lobes in order to minimize the heat distortion or deformation of the cam lobe during the heating of the lobes for hardening, has led to the use of high power density, short heat time, induction heating of the cam lobes preparatory to the subsequent hardening thereof by quenching. However, such high power density induction heating of the camshafts by the previously known camshaft induction heating and hardening systems presented certain problems in attaining an overall uniformity of hardness in the surface profile of the cam lobes. Thus, because of the relative close spacing of the cam lobes to one another along the camshaft, the peripheral edges of adjacent cam lobes experience stray induction heating during the induction heating of a given cam lobe for hardening. Previously hardened cam lobes thus are subject to drawing back or tempering of their hardened condition, leading to an undesirable decrease in hardness and in the uniformity thereof. While the use of flux shields in other induction heating applications to limit the effects of stray induction heating is well known, their use in conjunction with the extremely close spaced cam lobes of engine camshafts adversely affects the flux field of the cam lobe being heated by the inductor such as then results in the nonuniform heating and consequent nonuniform hardening thereof.
A high power density induction heating and hardening system for the cam lobes of a camshaft which overcomes the above mentioned limitations and disadvantages is disclosed in U.S. Pat. No. 4,604,510, assigned to the same assignee as that of the present application. In the system disclosed in this patent, a cam lobe which has just been inductively heated and quench hardened in the circular shape induction heating coil is additionally quench cooled by a supplementary coolant, during the interval the next succeeding cam lobe is being heated and quench hardened in the inductor, in order to thereby prevent the reheating and resulting drawback or tempering of the previously hardened cam lobe by stray induction heating thereof. Movable shields are provided to engage around the camshaft body between the adjoining hardened and unhardened cam lobes, after the camshaft has been properly indexed to position the unhardened next cam lobe in the induction heating coil, to thereby intercept and prevent the supplementary quenching coolant directed against the previously hardened cam lobe from impinging against the surface of the adjacent unhardened cam lobe being heated in the heating coil. Otherwise, owing to the short heating cycle, the surface of the cam lobe being heated in the inductor will not attain the required elevated temperature and uniformity for the desired proper hardening thereof.
Although the cam lobe induction heating and hardening system as disclosed in the above referred to U.S. patent operates satisfactorily to produce steel camshafts having hardened cam lobes of requisite hardness and with an acceptable degree of uniformity therearound, the lobes nevertheless are still characterized by a certain small amount of surface profile distortion and also nonuniformity in the hardness pattern depth. This latter condition is due mainly to the eccentric shape of the cam lobe that forms an inductive coupling gap with the circular shaped induction heating coil of irregular gap width therearound. As a result, the cam lobes are apt to become heated to a varying degree and depth around their periphery during the inductive heating of the cam lobe in the inductive heating coil. Also, the requirement for the coolant shields and their timed actuating mechanism adds to the overall cost of the camshaft hardening apparatus.